Restore affected estuaries: Manage extensive crab herbivory

Strategy:

The sustainability of estuaries is being challenged by anthropogenic activities, sea-level rise, more frequent storms due to climate change, and nutrient pollution. Extensive herbivory (plant eating behavior) and burrowing by the Purple Marsh Crab (Sesarma reticulatum) can also threaten an estuary’s stability and function.

Herbivory and burrowing by the Purple Marsh Crab is a documented driver of marsh “die off” events (marshes where more than 50% of the creek bank becomes unvegetated) in Massachusetts.1,2,3 Purple Marsh Crabs have been shown to conduct widespread grazing on the aboveground and belowground matter of the primary low marsh plant, cordgrass (Spartina alterniflora). The Marsh Crab creates burrows to hide from predators, to create a protected mating area, and for access to the belowground biomass. The Purple Marsh Crab’s herbivory and burrowing activities directly reduces marsh stability and habitat structure.

To restore impacted estuaries, communities need to assess how crab destabilizing activities have affected, and could affect, their local estuaries. Localized monitoring and management action plans can then be implemented in collaboration with town committees and government agencies.

Action

Assess and Monitor Purple Marsh Crab Activity
An important first step is to identify marsh edges that have been compromised due to Marsh Crab herbivory and burrowing so that “die off” areas can be monitored, mitigated, and possibly restored. In highly impacted areas, there can be as many as 50 crabs and 170 burrows per square meter (approximately 15 crabs and 52 burrow openings per square foot).3

The Marsh Crab’s aboveground herbivory reduces habitat for other species (such as snowy egrets and coastal fish) while belowground herbivory (from inside their burrows) compromises the structural integrity of the marsh by consuming the plant roots and rhizomes that stabilize marsh sediment. Extensive herbivory on root and rhizomes displaces sediment, destabilizes marshes, and reduces plant resiliency.3

Marsh Crabs prefer eating and burrowing in hard peat, which is prominently located on the edges of salt marshes; these are also the first areas impacted during storm events. Consequently, burrowing increases the vulnerability of salt marsh edge habitat to coastal erosion and storm events.1 Impaired edges also reduce the estuary’s ability to protect coastal communities from sea-level rise and stronger and more frequent storms projected under future climate change.

By assessing crab-induced herbivory to local marsh habitats and establishing a monitoring plan, communities can document changes in Marsh Crab presence while they work towards specific management actions to minimize and prevent “die off” events.

Displays a male mud Fiddler Crab (bottom left) and a Fiddler Crab burrow (top left) as well as a large Green Crab (top right) and two Green Crabs mating (bottom right). All photographs were at the Plum Island Estuary and used with permission by Amanda Davis.

Manage A Balanced Ecosystem
Damage from Marsh Crabs appears to be more widespread in marshes where top predators are less abundant.2,3 Marsh die-off sites appear to occur near popular recreational fishing areas where predators such as striped bass, blue crabs, and smooth dogfish are less abundant in comparison to vegetated marshes located further away from fishing areas.2

Another predator of Marsh Crabs, the invasive Green Crab (Carcinus maenas), may act as a biological manager; Green Crabs can outcompete Marsh Crabs by occupying their protective burrows and leaving them vulnerable and exposed to other predators.3 While the Green Crab is a populous and swarming invasive species? that can quickly devastate shellfish populations, they could possibly be managed to control Marsh Crab populations and ultimately assist in restoring the marsh’s stability.

Green Crabs are a nuisance and aggressive species that has been the culprit for the demise of bivalves. Therefore, if they were to be used as a biological manager of Marsh Crabs, it would need to be done carefully with impacts on other parts of the ecosystem in mind. To spread awareness and engage the community in Green Crab management, some groups (such as The Green Crab R&D Project) and restaurants are highlighting Green Crab’s potential as food, commonly as an ingredient for seafood stock. Another management initiative is to encourage fishermen to use them as fishing bait to target Tautog (also known as Blackfish). Crab management is difficult because of the large number of crabs that exist in estuaries and because these animals are changing their geographical range, likely due to increased water temperature from climate change.4,5

What happens when Wells National Estuarine Research Reserve researchers leave a time-lapse camera in a crab trap with 300+ invasive green crabs?

The Future of the Fiddler Crab
Similar to Green Crabs, mud Fiddler Crabs (Uca pugnax) create burrows in salt marshes and may also outcompete and utilize Marsh Crabs burrows. Fiddler Crab burrowing activity in die-off areas can prevent seedlings from germinating and their burrowing has been connected to further erosion and elevation loss.6 Warmer ocean temperatures due to climate change have allowed several crab species to expand their geographical range northwards, including Fiddler crabs, which are now found in northern Massachusetts and Gulf of Maine estuaries.4,5 Since mud Fiddler Crabs have historically been absent from this region, management strategies are open-ended because it is unclear if their newly established presence and interactions with other crab species will have positive or negative impacts on native populations and estuary function; however, Fiddler Crabs are being actively observed and monitored by researchers from the Virginia Institute of Marine Science (VIMS) and Massachusetts Coastal Zone Management (CZM).